There is an ever increasing demand for magnetic recording media with higher storage capacity, lower noise and lower costs. Recording densities in computers have increased steadily over the last two decades.
Magnetic discs and disc drives provide quick access to vast amounts of stored information. Both flexible and rigid discs are available. Data on the discs is stored in circular tracks and divided into segments within the tracks. Disc drives typically employ one or more discs rotated on a central axis. A magnetic head is positioned over the disc surface to either access or add to the stored information. The heads for disc drives are mounted on a movable arm that carries the head in very close proximity to the disc over the various tracks and segments. The structure of disc drives is well known.
The structure of a typical thin film disk is multilayered and includes a substrate at its base covered by an underlayer, a magnetic layer and optionally, an overlayer at the top. The overlayer maybe coated with an overcoat and an organic lubricant. The magnetic layer is the main body on which the magnetic bits are recorded. Longitudinal recording media comprised of cobalt or cobalt alloy-based magnetic films having a chromium or chromium alloy-based underlayer deposited on a nonmagnetic substrate have become the industry standard.
Important magnetic properties, such as coercivity (Hc), remanent magnetization (Mr) and coercive squareness (S*), which are crucial to the recording performance of the Co alloy thin film for a fixed composition, depend primarily on its microstructure. For thin film longitudinal magnetic recording media, the desired crystalline structure of the Co and Co alloys is hexagonal close packed (HCP) with uniaxial crystalline anisotropy and a magnetization easy direction along the c-axis is in the plane of the film. The better the in-plane c-axis crystallographic texture, the higher the coercivity of the Co alloy thin film used for longitudinal recording. This is required to achieve a high remanence. For very small grain sizes coercivity increases with increased grain size. Large grains, however, results in greater noise. There is a need to achieve high coercivities without the increase in noise associated with large grains. To achieve a low noise magnetic medium, the Co alloy thin film should have uniform small grains with grain boundaries which can magnetically isolate neighboring grains. This kind of microstructure and crystallographic texture is normally achieved by manipulating the deposition process, by grooving the substrate surface, or most often by the proper use of an underlayer.
Underlayers can strongly influence the crystallographic orientation, the grain size and chemical segregation at the Co alloy grain, boundaries. Underlayers which have been reported in the literature include Cr, Cr with an additional alloy element X (X=C, Mg, Al, Si, Ti, V, Co, Ni, Cu, Zr, Nb, Mo, La, Ce, Nd, Gd, Tb, Dy, Er, Ta, and W), Ti, W, Mo, and NiP. While there would appear to be a number of underlayer materials available, in practice, only a very few work well enough to meet the demands of the industry. Among them, the most often used and the most successful underlayer is pure Cr. For high density recording, in plane orientation has heretofore been achieved by grain-to-grain epitaxial growth of the HCP Co alloy thin film on a body centered cubic (BCC) Cr underlayer. The polycrystalline Co-based alloy thin film is deposited with its c-axis, the [0002] axis, either parallel to the film plane or with a large component of the c-axis in the film plane. It has been shown-by K. Hono, B. Wong, and D. E. Laughlin, xe2x80x9cCrystallography of Co/Cr bilayer magnetic thin filmsxe2x80x9d, Journal of Applied Physics 68 (9) p. 4734 (1990), that BCC Cr underlayers promote grain-to-grain epitaxial growth of HCP Co alloy thin films deposited on these underlayers. The heteroepitaxial relationships between Cr and Co which bring the [0002]Co axis down or close to the film plane are (002)Cr//(110)Co, (110)Cr//(101)Co, (110)Cr//(100)Co, and (112)Cr//(100)Co. Different Co/Cr epitaxial relationships prevail for different deposition processes. To obtain a good BCC structure which promotes the formation of the HCP structure, the Cr underlayer must be thicker than about 100 A. U.S. Pat. No. 4,652,499 discloses efforts to improve the underlayer by adding vanadium (V) to Cr to change its lattice constant and thereby to promote a better lattice matching between the HCP Co alloys, CoPt or CoPtCr, and the BCC CrV underlayer.
In perpendicular magnetic recording media, a thin film layer which is sometimes called a precoat or a seed layer, is commonly deposited between the substrate and the underlayer to isolate the underlayer from possible substrate contaminants. Materials used for this layer with varying degrees of success include Al, Ti, Ni3P, TiSi2, Cr, C, Ta, W and Zr. A. Nakamura and M. Futamoto, xe2x80x9cEpitaxial Growth of Co/Cr Bilayer Films on MgO Single Crystal Substratesxe2x80x9d, J. Applied Physics, Vol. 32, part 2, No. 10A, L1410 (October, 1993) describes a Co film deposited on a single crystal MgO (002). A (110) bicrystalline longitudinal magnetic recording medium, CoCrPt/Cr, formed on a MgO single crystal disk substrate is described in M. Futamoto et al., xe2x80x9cMagnetic and recording characteristic of bicrystalline longitudinal recording media formed on an MgO single crystal disc substratexe2x80x9d, IEEE Transactions on Magnetics, Vol. 30, No. 6, p.3975 (1994). Because of the anisotropic magnetic properties and hence, the recording properties, around the circumference of the single crystal disk, this disk is of limited utility.
The need for lighter, smaller and better performing computers with greater storage density demands higher density hard disk media. It is an object of the present invention to meet those demands with a longitudinal magnetic recording media having high coercivity and low noise.
The present invention provides a recording media incorporated in a disc drive having a rotatable disc for operation in conjunction with magnetic transducing heads for the recording and reading of magnetic data. The improved recording media optionally has a novel seed layer on which to induce the (002) crystallographic texture in an underlayer for the magnetic film of the media. The magnetic recording medium of the invention is comprised of a substrate, a magnetic layer, preferably formed from Co or Co alloy film, an optional seed layer, preferably comprised of a material having a B1-ordered crystalline structure with a (002) texture, such as MgO, sputter deposited on the substrate. An underlayer is also provided which is comprised of a material having a body centered-cubic derivative crystalline structure disposed between the substrate and the magnetic layer. The body centered cubic derivative crystalline structure may be selected from the group consisting of B2, DO3 and L21. Examples of suitable materials include NiAl, AlCo, FeAl, FeTi, CoFe, CoTi, CoHf, CoZr, NiTi, CuBe, CuZn, AlMn, AlRe, AgMg, Mn3Si and Al2FeMn2, and is most preferably Mn3Si, FeAl or NiAl.
The underlayer may be formed in multiple layers wherein each layer is a different one of the foregoing materials. The Co or Co alloy used as the magnetic layer has a hexagonal close packed (HCP) structure and is deposited with its c-axis, the magnetic easy axis (the direction in which it is easily magnetized), substantially parallel to the plane of the magnetic layer.
The recording medium may also include a chromium or chromium alloy intermediate layer interposed between the magnetic layer and the underlayer. The intermediate layer is relatively thin, preferably between about 10 to 500 A. Thinner layers can provide some improvement provided the layer is thick enough to form a substantially continuous layer on the surface of the underlayer.
The magnetic layer may be covered by an overlayer which in turn may be covered by an overcoat. An organic lubricant is preferably added over the overcoat.
In an alternative arrangement, there may be a first magnetic layer and a second magnetic layer with one or more interlayers disposed between the two magnetic layers. The interlayer is typically Cr, of about 10 to 40 A. The second magnetic layer may be covered by the overlayer/overcoat/lubricant layers described above.
Materials with the B2, DO3 and L21 structures are ordered structural derivatives of the body centered cubic (BCC) structure, which is the structure of Cr. A derivative structure of a basic structure is one in which one or more symmetry elements (translational or orientational) is (are) suppressed. The basic periodicity and position of the atoms remains the same but the specific atomic occupancies change. BCC structures have many xe2x80x9cderivativexe2x80x9d structures, including, but not limited to the B2, DO3 and L21. See FIG. 35. The BCC structure has two atoms in its unit cell. The occupancy of the atom at (000) and that at (1/2, 1/2, 1/2) is the same. For the B2 structure, however, the atomic occupancy is different at (000) and (1/2, 1/2, 1/2). The same can be seen to be true for the other examples of derivative structures. The degree of order goes up from BCC to B2 to DO3 to L21.
NiAl, for example, is a Hume-Rothery xcex2-phase electron compound with a valence electron/atom ratio of 3/2 which gives a B2 crystalline structure, shown in FIG. 2(b). NiAl has a lattice constant of 0.2887 nm, almost identical to that of Cr, 0.2884 nm. By placing the B2-ordered structure on the substrate, below the magnetic layer, the Co or Co alloy films, when deposited, either directly or through an intermediate Cr or Cr alloy layer, grow epitaxially at the B2 interface and reorient the HCP c-axis away from being random or normal to the film plane, yielding a stable, improved thin film microstructure with magnetic properties which are particularly well suited to high density recording. NiAl, for example, maintains its B2 structure stable up to the melting point of 1911 K and the structure does not change over a wide composition range from 41.5 to 55 at % Al at 673 K. Strong bonding between the metallic atoms slows the atomic mobility during the film""s deposition thereby yielding a smaller grain size film than is present with the Cr underlayer. A small grain size can benefit the recording properties of the media by increasing the number of grains per unit area and hence, lowering the media noise.
In addition, NiAl is nonmagnetic with an extremely low magnetic susceptibility, on the order of 2xc3x9710xe2x88x927 to 5xc3x9710xe2x88x927 emu/g, has good thermal conductivity, high stiffness, and good environmental corrosion resistance.
The lattice parameter of Mn3Si is 5.72 xc3x85, which is twice as large as that of NiAl and Cr underlayers. Thus, the lattice constants of the material chosen for the underlayer should be close to that of Cr or integer multiples thereof. Underlayers made of materials having lattice parameters that give rise to epitaxial growth, i.e, atomic matching across the interface of the crystal layers, will provide the desired results. DO3 Mn3Si phase is stable at 677xc2x0 C.